<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #app{
      width:100%;
      height: 30px;
      box-sizing: inherit;
      display: inline-block;
    }
    .item{
      height:100%;
      width:150px;
      border: 1px red solid;
      display: inline-block;
    }
  </style>
</head>
<body>
  <div id="app">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
</body>
</html>